Introspection

In this part of the Python programming tutorial, we will talk about introspection.

Introspection is an act of self examination. In computer programming, introspection is the ability to determine the type of an object at runtime. Python programming language has a large support of introspection. Everything in Python is an object. Every object in Python may have attributes and methods. By using introspection, we can dynamically inspect Python objects.

The dir() function

The dir() function is the most important function when doing introspection. The function returns a sorted list of attributes and methods belonging to an object.

>>> dir(())
['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', 
'__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', 
'__getslice__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', 
'__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', 
'__reduce_ex__', '__repr__', '__rmul__', '__setattr__', '__sizeof__', 
'__str__', '__subclasshook__', 'count', 'index']

Here we see an output of the dir() function for a tuple object.

>>> print ().__doc__
tuple() -> empty tuple
tuple(iterable) -> tuple initialized from iterable's items

If the argument is a tuple, the return value is the same object.

Our investigation showed, that there is a __doc__ attribute for a tuple object.

#!/usr/bin/python

# dir.py

import sys

class Object:
   def __init__(self):
      pass
   def examine(self):
      print self


o = Object()

print dir(o)
print dir([])
print dir({})
print dir(1)
print dir()
print dir(len)
print dir(sys)
print dir("String")

The example examines several objects using the dir() function. A user defined object, native data types, a function, a string or a number.

Without any argument, the dir() returns names in the current scope.

>>> dir()
['__builtins__', '__doc__', '__name__', '__package__']
>>> import sys
>>> import math, os
>>> dir()
['__builtins__', '__doc__', '__name__', '__package__', 'math', 'os', 'sys']

We execute the dir() function before and after we include some modules.

The type(), id() functions

The type() function returns the type of an object.

#!/usr/bin/python

# types.py

import sys


def function(): pass

class MyObject():
   def __init__(self):
      pass

o = MyObject()

print type(1)
print type("")
print type([])
print type({})
print type(())
print type(object)
print type(function)
print type(MyObject)
print type(o)
print type(sys)

The example print various types of objects to the console screen.

$ ./types.py 
<type 'int'>
<type 'str'>
<type 'list'>
<type 'dict'>
<type 'tuple'>
<type 'type'>
<type 'function'>
<type 'classobj'>
<type 'instance'>
<type 'module'>

Output of the types.py script.

The id() returns a special id of an object.

#!/usr/bin/python

# ids.py

import sys

def fun(): pass

class MyObject():
   def __init__(self):
      pass

o = MyObject()

print id(1)
print id("")
print id({})
print id([])
print id(sys)
print id(fun)
print id(MyObject)
print id(o)
print id(object)

The code example prints ids of various objects. Both built-in and custom.

The sys module

The sys module provides access to system specific variables and functions used or maintained by the interpreter and to functions that interact strongly with the interpreter. The module allows us to query about the Python environment.

>>> import sys
>>> sys.version
'2.7.2+ (default, Oct  4 2011, 20:03:08) \n[GCC 4.6.1]'
>>> sys.platform
'linux2'
>>> sys.path
['', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-linux2', 
'/usr/lib/python2.7/lib-tk', '/usr/lib/python2.7/lib-old', 
'/usr/lib/python2.7/lib-dynload', '/usr/local/lib/python2.7/dist-packages', 
'/usr/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages/PIL', 
'/usr/lib/python2.7/dist-packages/gst-0.10', '/usr/lib/python2.7/dist-packages/gtk-2.0', 
'/usr/lib/python2.7/dist-packages/ubuntu-sso-client']

In the above code we examine the Python version, platform and search path locations.

We will have another four variables of the sys module. We can use the dir() function to get a full list of variables and functions of the sys module.

>>> sys.maxint
2147483647
>>> sys.executable
'/usr/bin/python'
>>> sys.argv
['']
>>> sys.byteorder
'little'

The example presents maxint, executable, argv and byteorder attributes of the sys module.

>>> sys.maxint
2147483647

The maxint is the largest positive integer supported by Python's regular integer type.

>>> sys.executable
'/usr/bin/python'

The executable is a string giving the name of the executable binary for the Python interpreter, on systems where this makes sense.

>>> sys.argv
['']

This gives a list of command line arguments passed to a Python script.

>>> sys.byteorder
'little'

The byteorder is an indicator of the native byte order. This will have the value 'big' on big-endian (most-significant byte first) platforms, and 'little' on little-endian (least-significant byte first) platforms.

Various

Next we will show various other ways of inspecting our objects.

#!/usr/bin/python

# attr.py

def fun(): pass


print hasattr(object, '__doc__')
print hasattr(fun, '__doc__')
print hasattr(fun, '__call__')

print getattr(object, '__doc__')
print getattr(fun, '__doc__')

The hasattr() function checks, if an object has an attribute. The getattr() function returns the contents of an attribute, if there are some.

$ ./attr.py 
True
True
True
The most base type
None

The isinstance function checks, if an objects is an instance of a specific class.

>>> print isinstance.__doc__
isinstance(object, class-or-type-or-tuple) -> bool

Return whether an object is an instance of a class or of a subclass thereof.
With a type as second argument, return whether that is the object's type.
The form using a tuple, isinstance(x, (A, B, ...)), is a shortcut for
isinstance(x, A) or isinstance(x, B) or ... (etc.).

We can get the describtion of a function interactively.

#!/usr/bin/python

# instance.py


class MyObject():
   def __init__(self):
      pass

o = MyObject()

print isinstance(o, MyObject)
print isinstance(o, object)
print isinstance(2, int)
print isinstance('str', str)

As we know, everything is an object in Python. Even numbers or strings. The object is a base type of all objects in Python. That is why isinstance(o, object) returns True.

$ ./instance.py 
True
True
True
True

The issubclass() function checks, if a specific class is a derived class of another class.

#!/usr/bin/python

# subclass.py

class Object():
   def __init__(self):
      pass

class Wall(Object):
   def __init__(self):
      pass

print issubclass(Object, Object)
print issubclass(Object, Wall)
print issubclass(Wall, Object)
print issubclass(Wall, Wall)

In our code example, the Wall class is a subclass of the Object class. Interestingly, Object and Wall are subclasses of themselves. The Object class is not a subclass of class Wall.

$ ./subclass.py 
True
False
True
True

The __doc__ attribute gives some documentation about an object and the __name__ attribute holds the name of the object.

#!/usr/bin/python

# namedoc.py

def noaction():
   '''A function, which does nothing'''
   pass


funcs = [noaction, len, str]

for i in funcs:
   print i.__name__
   print i.__doc__
   print "-" * 75

In our example, we crete a list of three functions. One custom and two native. We go through the list and print the __name__ and the __doc__ attributes.

$ ./namedoc.py
noaction
A function, which does nothing
---------------------------------------------------------------------------
len
len(object) -> integer

Return the number of items of a sequence or mapping.
---------------------------------------------------------------------------
str
str(object) -> string

Return a nice string representation of the object.
If the argument is a string, the return value is the same object.
---------------------------------------------------------------------------

Output.

Finally, there is also a callable() function. The function checks, if an object is a callable object. Or in other words, if an object is a function.

#!/usr/bin/python

# callable.py

class Car:
      
    def setName(self, name):
        self.name = name    

def fun():
    pass

c = Car()    
    
print callable(fun)
print callable(c.setName)
print callable([])
print callable(1)

In the code example we check if three objects are callables.

print callable(fun)
print callable(c.setName)

The fun() function and the setName() method are callables. (A method is a function bound to an object.)

$ ./callable.py
True
True
False
False

In this part of the Python tutorial, we have talked about introspection in Python. More tools for doing introspection can be found in the inspect module.

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